Biomechanical Properties of Hemlocks: A Novel Approach to Evaluating Physical Barriers of the Plant-Insect Interface and Resistance to a Phloem-Feeding Herbivore.

Paul Ayayee,Lynne Rieske,Fuqian Yang

doi:10.3390/insects5020364

Abstract

Micromechanical properties that help mediate herbivore access may be particularly important when considering herbivorous insects that feed with piercing-sucking stylets. We used microindentation to quantify the micromechanical properties of hemlock, Tsuga spp., to quantify the hardness of the feeding site of the invasive hemlock woolly adelgid, Adelges tsugae. We measured hardness of the hemlock leaf cushion, the stylet insertion point of the adelgid, across four seasons in a 1 y period for four hemlock species growing in a common garden, including eastern, western, mountain, and northern Japanese hemlocks. Leaf cushion hardness was highest in the fall and winter and lowest in summer for all species. Northern Japanese hemlock had relatively greater hardness than the remaining species. Our data contributes an additional perspective to the existing framework within which greater susceptibility and subsequent mortality of eastern hemlocks is observed. The potential application of microindentation to understanding the nature and relevance of plant mechanical defenses in plant–herbivore interactions is also demonstrated and highlighted.

Highlights

Plants employ a range of constitutive and induced defenses to deter insect herbivory
We demonstrate temporal changes in hemlock leaf cushion hardness, but these differences differ from the seasonal increases in tissue hardness evident in some systems [5,9,11,12,17]
Leaf cushion hardness across hemlock species was lowest in the summer and greatest in the autumn, perhaps reflecting tissue maturation following spring growth and summer dormancy [40]; leaf cushion hardness was intermediate in the winter and spring

Summary

Introduction

Plants employ a range of constitutive and induced defenses to deter insect herbivory. Obvious issues with penetrometer readings include lack of a standardized process for quantifying target measurements, lack of detail in construction, superficial use of the measures generated, and the realization that what is measured is not technically ‘toughness’ [13,20,21] Alternatives such as razor slicing have been used to estimate leaf fracture toughness [21], but both approaches have drawbacks [14,21] that limit their ability to reliably measure the biomechanical properties relevant in insect-plant relations. In this study we investigated the application of microindentation to elucidate the role of biomechanical properties toward resistance in hemlocks to adelgid herbivory, and quantified species-specific and seasonal differences in hardness of the adelgid feeding site for four hemlock species over a one-year period. We sought to determine whether the susceptibility of eastern hemlock to the hemlock woolly adelgid might be influenced by micromechanical properties of the adelgid stylet insertion point

Experimental Design

Sampling

Microindentations

Statistical Analysis

Results and Discussion

Discussion and Conclusions